1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to an apparatus and a method for antenna verification in a mobile communication terminal employing closed loop mode transmit diversity.
2. Description of the Related Art
Transmit diversity is a technique for increasing reception performance, in such a manner that a transmitting side transmits a signal using at least two antennas and a receiving side combines the signals received from the antennas. The transmit diversity enables the receiving side to obtain a higher gain by changing an amplitude, a phase or a symbol in the patterns of the antennas.
The transmit diversity is largely classified into an open loop mode transmit diversity and a closed loop mode transmit diversity according to whether or not a signaling of a transmission pattern exists. A wideband code division multiple access (WCDMA) system employs transmit diversity which uses two antennas in a base station, and supports two open loop mode transmit diversity schemes and closed loop mode transmit diversity.
The closed loop mode transmit diversity is classified into two modes. Mode 1 changes the phase pattern, and mode 2 changes the phase and amplitude patterns. According to such closed loop mode transmit diversity, a terminal selects a pattern to enlarge the intensity of a reception signal and then notifies a base station of the selected pattern using a feedback information (FBI) field of an uplink dedicated physical control channel (DPCCH), and the base station applies the transmit diversity according to the pattern reported from the terminal.
As an example, mode 1 of the closed loop mode transmit diversity will be described with reference to the 3GPP TS 25.214 V6.4.0 technical specification published on December 2005. In order to obtain the maximum reception signal, a terminal selects phase patterns ‘Pi’ for the ith slots by using fading information which has been obtained in a reception end using a primary common pilot channel (CPICH) for reception. The terminal inserts a feedback message (FSM) into the FBI field of the uplink DPCCH as shown in Table 1 below and then transmits the uplink DPCCH. Table 1 shows that the FSM is inserted as ‘0’ or ‘1’ into the FBI field of the uplink DPCCH according to phase patterns ‘Pi’ selected for the ith slots. The calculation method for the phase patterns ‘Pi’ has been described in detail in annex ‘A.2’ of the 3GPP TS 25.214 V6.4.0 technical specification.
TABLE 1Slot #01234567891011121315FSM00 π/20 π/20 π/20 π/20 π/20 π/20 π/201π−π/2π−π/2π−π/2π−π/2π−π/2π−π/2π−π/2π
The base station then receives the FSM through the uplink DPCCH, and generates the antenna complex vectors ‘w1’ and ‘w2’ on the basis of the received FSM, which will be applied to antennas #1 and #2 of a transmitter in the base station, respectively. The antenna complex vectors ‘w1’ and ‘w2’ are defined as shown in Equations 1 and 2 below.
                              w          1                =                  1                      2                                              (        1        )                                          w          2                =                                                            ∑                                  i                  =                                      n                    -                    1                                                  n                            ⁢                              cos                ⁡                                  (                                      ϕ                    i                                    )                                                      2                    +                      j            ⁢                                                            ∑                                      i                    =                                          n                      -                      1                                                        n                                ⁢                                  sin                  ⁡                                      (                                          ϕ                      i                                        )                                                              2                                                          (        2        )            
In equation 2, φiε{0, π, π/2, −π/2}, and ‘i’ represents an index number for an ith slot in one frame.
In the base station, a dedicated physical channel (DPCH) signal to be transmitted through antennas #1 and #2 is spread/scrambled, and then is divided into two signals so as to correspond to antennas #1 and #2. In the two divided signals, the signal corresponding to antenna #1 is multiplied by the antenna complex vector ‘w1’, and the signal corresponding to antenna #2 is multiplied by the antenna complex vector ‘w2’. The two signals multiplied by the antenna complex vectors ‘w1’ and ‘w2’, are added to a CPICH1 and a CPICH2, respectively, and then are transmitted through antennas #1 and #2.
In order to demodulate signals to which the transmit antenna diversity is applied as described above, the terminal multiplies the DPCH by the antenna complex vectors ‘w1’ and ‘w2’ which the base station has used in the DPCH transmission. The antenna complex vectors ‘w1’ and ‘w2’ have values transmitted from the terminal to the base station through the uplink DPCCH as described above.
In the case of using the transmit diversity, if the terminal cannot correctly read a transmission pattern (i.e. the phase/amplitude/symbol of a signal transmitted from the base station), the reception performance of the terminal may be of substandard quality as compared with the case of not using any diversity. For this reason, in the case of employing the closed loop mode transmit diversity, in order to prevent the performance deterioration caused by an error in the patterns, the terminal performs a procedure called ‘antenna verification’ in which the terminal again verifies the pattern transmitted from the base station and performs demodulation.
A poor radio environment can cause an error in the uplink DPCCH. When an error occurs in the FSM transmitted from the terminal to the base station, the base station will perform a transmit diversity using incorrect vector values, not using the antenna complex vectors ‘w1’ and ‘w2’ desired by the terminal. When a transmit diversity using wrong vector values is applied due to an error in the FSM transmitted from the terminal as described above, the reception performance of the terminal deteriorates.
In order to prevent performance deterioration due to a feedback error as described above, the terminal employs an algorithm which enables the reception unit of the terminal to estimate the antenna complex vectors applied in the base station. Such a procedure for estimating the antenna complex vectors applied to the transmit diversity in the base station is called ‘antenna verification’. The ‘antenna verification’ has been described in detail in annex ‘A.1’ of the 3GPP TS 25.214 V6.4.0 technical specification.
There are various algorithms used for the antenna verification. Most of the algorithms basically use the power and the signal-to-noise ratio of a DPCCH pilot signal received in the terminal, and an uplink error rate, in order to estimate antenna complex vectors applied to transmit diversity in the base station. The terminal performs antenna verification using this information, and demodulates a received signal, by using the antenna complex vectors estimated to have been applied to the transmit diversity in the base station as the antenna complex vectors ‘w1’ and ‘w2’.
In the information used in the antenna verification as described above, the power and the signal-to-noise ratio of a DPCCH pilot signal received in the terminal can be estimated from the primary CPICH. Also, it has been generally understood that the uplink error rate is about 4%, which is a target value in network design.
Since the uplink error rate used in the antenna verification is only a target value, the actual uplink error rate may increase depending on the limitation of transmission power when the terminal enters a cell boundary area or a shadow area. In this case, the actual uplink error rate can exceed the uplink error rate established as a target value in design. When the antenna verification algorithm is continuously performed using the target uplink error rate, the accuracy of the antenna verification algorithm deteriorates, thereby increasing performance deterioration due to the transmit diversity. Such a phenomenon is equally caused in both modes 1 and 2 of the closed loop mode transmit diversity.